The in-game Codex of the Multiverse doesn't list all available formats. When I go to challenge my friend, there's a TON of extra formats there, like "Historic Rebalanced". What does that mean? WHO KNOWS. It's not on the webpage for Arena formats https://magic.wizards.com/en/news/mtg-arena/mtg-arena-formats and it's not in the in-game codex.